Caption: T lymphocytes and cancer cell. Coloured scanning electron micrograph (SEM) of T lymphocyte cells (smaller round cells) attached to a cancer cell. T lymphocytes are a type of white blood cell and one of the components of the body's immune system. They recognise a specific site (antigen) on the surface of cancer cells or pathogens and bind to it. Some T lymphocytes then signal for other immune system cells to eliminate the cell. Cytotoxic T lymphocytes eliminate the cell themselves by releasing a protein that forms pores in the cell's membrane. The genetic changes that cause a cell to become cancerous lead to the presentation of tumour antigens on the cell's surface. Magnification: x2500 at 10 centimetres wide.